The circuit presented here
can be used for connecting two telephones in parallel
and also as a 2-line intercom.
Usually a single telephone
is connected to a telephone line. If another telephone
is required at some distance, a parallel line is taken
for connecting the other telephone. In this simple parallel
line operation, the main problem is loss of privacy
besides interference from the other phone. This problem
is obviated in the circuit presented here.
Under normal condition, two
telephones (telephone 1 and 2) can be used as intercom
while telephone 3 is connected to the lines from exchange.
In changeover mode, exchange line is disconnected from
telephone 3 and gets connected to telephone 2.
For operation in intercom mode,
one has to just lift the handset of phone 1 and then
press switch S1. As a result, buzzer PZ2 sounds. Simultaneously,
the side tone is heard in the speaker of handset of
phone 1. The person at phone 2 could then lift the handset
and start conversation. Similar procedure is to be followed
for initiation of the conversation from phone 2 using
switch S2. In this mode of operation, a 3-pole, 2-way
slide-switch S3 is to be used as shown in the figure.
In the
changeover mode of operation, switch S3 is used to changeover
the telephone line for use by telephone 2. The switch
is normally in the intercom mode and telephone 3 is
connected to the exchange line. Before changing over
the exchange line to telephone 2, the person at telephone
1 may inform the person at telephone 2 (in the intercom
mode) that he is going to changeover the line for use
by him (the person at telephone 2). As soon as changeover
switch S3 is flipped to the other position, 12V supply
is cut off and telephones 1 and 3 do not get any voltage
or ring via the ring-tone-sensing unit.
Once switch S3 is flipped over
for use of exchange line by the person at telephone
2, and the same (switch S3) is not flipped back to normal
position after a telephone call is over, the next telephone
call via exchange lines will go to telephone 2 only
and the ring-tone-sensing circuit will still work. This
enables the person at phone 3 to know that a call has
gone through. If the handset of telephone 3 is lifted,
it is found to be dead. To make telephone 3 again active,
switch S3 should be changed over to its normal position. |
